Stents are small, intricately cut tubes, generally made of materials such as stainless steel. Cardiovascular stents, are permanently placed in a blood vessel to act as scaffolding to keep an occluded artery open. In use, cardiovascular stents are inserted into the artery on a catheter and are typically deployed by expanding a very small balloon at the end of the catheter upon which the stent is mounted.
Cardiovascular stents must meet stringent requirements to work properly. If the stent contains any rough or sharp edges, it will damage blood cells or the blood vessel in which it is inserted. This can lead to further atherosclerotic plaquing, emboli or thrombi, and result in potentially life threatening situations. This invention relates to an illumination and inspection system for stents, and other similar parts that take the form of a small precisely machined tube. This invention also relates to image processing software techniques optimized for inspection of such tubes as well as a wide variety of other parts with highly repetitive features.
Lasers are typically used to cut stents. This process, while highly precise, can occasionally produce defective parts. Stents tend to be fairly small, with diameters approximating 1 mm. After processing, the individual cut features on a stent range from 50 to 200 microns in size. Accordingly, small changes in manufacturing process parameters such as laser power, tubing diameter, or mechanical jitter can cause defects. Such defects may include an out of tolerance feature size or a malformed feature.
Since stents are used in the heart and other critical areas of blood flow, a failure in the function of the stent could be life threatening. Thus, manufacturers of stents typically employ 100% inspection procedures. A human operator utilizing a 50×optical power stereo-microscope typically inspects for visual defects. Dimensional inspection is typically done by a human operator utilizing a Profile Projector, such as the V-12 made by Nikon Inc. of Japan. Alternatively, this inspection can be done automatically by utilizing a vision system such as the SmartScope made by Optical Gauging Products of Rochester, N.Y., or the Ram Optical OMIS made by Newport Corp. of Irvine, Calif.
The problems associated with either the manual or automatic approaches to inspection are numerous. First, human error makes visual inspection of products less than completely effective. Also, such manual inspection is relatively slow and thus a relatively costly aspect of the manufacturing process. Furthermore, the pass/fail criteria of the profile projector using overlays, as is typically employed in manual inspection, does not generally provide any numeric dimensional data that would otherwise be useful for process control.
While the possibility exists to employ automated systems, automated vision systems also have similar and other problems. First, the use of automated vision systems, like human inspection, tends to be slow. Such systems utilize a standard NTSC RS-170 style video camera that images a flat field onto the sensor. Since stents are cylindrically shaped, only a small section of the stent can be in focus at any one time. Also, since stents tend to be long and thin, a camera can only view a small section of the stent at sufficient magnification to effectively perform inspection. Typically, inspection with these systems involves programming the system to move up and down the length of a stent mounted on a mandrel. Such inspection involves looking at a small field of view, usually approximately 2 mm at a time. After inspecting the length of the stent, it is then rotated on a mandrel and the process is repeated. This can result in inspection cycle times of ten minutes or more.
In addition to problems in speed, these vision systems also have difficulty with accuracy. Because stents are electro-polished after being laser cut, the surfaces of the stent have a highly reflective mirrorlike finish, and rounded contours. Current illumination systems either use a fiber optic ring light or a sapphire rod or “mandrel” which acts as a backlight. Since the stents are highly reflective, intense hot spots and glare on the image can cause false or inaccurate measurements.
In an attempt to overcome this problem, sapphire rod illuminators have been employed. Such rods are first frosted to provide an even diffuse surface. The ends of the rod are optically polished to allow light, typically from a fiber optic bundle, to enter either end of the rod. The tubular stent is placed on this rod and the rod acts like a backlight source.
While, overall, the sapphire rod approach probably results in an improvement over the results obtainable with a ring light, sapphire rod illuminators have their own set of problems. To a camera looking down, the stent appears dark against the bright background of the sapphire mandrel. One problem that frequently occurs due to the highly polished surface and curved profile of the stent, or if the stent has slanting side walls, is that the walls themselves can be illuminated and appear as bright as the background sapphire which in turn makes the stent appear smaller than it actually is. This error can be as much as 25 microns, the manufacturing tolerance band for many stents.
In addition to measuring the width of a stent section, commonly known as a strut, stent manufacturers also measure the wall thickness of the stent at various locations along its length. Current manual and automatic systems can be used to measure wall thickness, but problems arise in the accuracy and repeatability of the current methods. A vision system can look at the edge of the sidewall of a stent and measure its width. Again glare and uneven illumination from a fiber optic ring light make it difficult to properly image a stent. On densely cut stents it can be hard to find an area on the stent that is open enough to view the sidewall while looking down on stent along its length.
Contact methods utilizing a micrometer are also generally problematic for measuring wall thickness. Stent features are quite small and the micrometer is a handheld device more readily used for measuring larger parts. Again this is a time consuming manual method and would benefit from automation.